Some examples of simple groups based on interest:
Simple groups can be either public or private. Public groups are listed in your members' accounts and they can opt in to receiving emails that are sent to this group. It would be appropriate to use a public group for your Improvers, as an individual's perception of their own level of ability will determine whether or not they want to receive information aimed at this group.
Private groups are appropriate for groups where the membership is determined by election or appointment. For example, a member cannot opt in to membership of the committee, so the maintenance of this group needs to be controlled by the club administrator.